1. Field of Invention
The invention relates to a fuel cell system which has a high temperature fuel cell stack with current diverters and a reformer and/or an afterburner, the current diverters on making contact with low temperature connecting elements.
2. Description of Related Art
Fuel cells are used for direct electrochemical conversion of a combustion gas and an oxidizer into the corresponding reaction products with release of electrical energy. The combustion gas and the oxidizer are collectively called the working stock. A fuel cell system is generally an arrangement which, in addition to one or more fuel cells which are interconnected into a so-called fuel cell stack, has other components such as, for example, a reformer, afterburner or electronic units for control of the system or for conversion of the generated voltage. A reformer is used to produce hydrogen-containing combustion gas from generally liquid, easily handled and readily available fuel, such as, for example, gasoline or diesel fuel which is supplied to the anode of the fuel cell. Fuel cells, in general, do not completely convert the combustion gas. The reaction products are then conventionally supplied to an afterburner where they are burned before they are released as an exhaust gas into the environment.
There are different types of fuel cells which differ in the electrolyte used for ion conduction. The temperature at which the fuel cell can be operated depends on the electrolyte material used. The solid oxide fuel cell (SOFC), which is operated at 800° C. to 1000° C., and the molten carbonate fuel cell (MCFC), which is operated at roughly 650° C., have the highest operating temperatures. These two types of fuel cells are collectively called high temperature fuel cells (HTFC) below.
To carry off the current produced by the fuel cells, current diverters lead from the fuel cell stack to the connecting elements to which, frequently, an electronic converter unit (DC-DC converter) is connected for conversion of the voltage, and optionally, for its stabilization. The temperature of the connecting elements should not exceed a given maximum value which is roughly 100° C. in order not to overheat the connected converter unit. Thus, one end of the current diverter has the high temperature of the HTFC, while the other end is kept at the lower temperature of the connecting element.
The high electrical conductivity which is desired for the current conduction of the diverters is accompanied by a correspondingly high thermal conductivity according to the Wiedemann-Franz law. The great temperature difference between the ends of the current diverters thus leads to conduction of heat from the high temperature fuel cell stack to the low temperature connecting elements. On the connecting elements, this heat is ordinarily dissipated by suitable cooling measures in order to prevent overheating of the connecting elements above the maximum allowable temperature. On the fuel cell stack, the removed heat results in local cooling (cold spot) in the vicinity of the connecting points. These cold spots are disadvantageous for efficient operation of the fuel cells. On the one hand, fuel utilization is less effective on the cold spot itself, and on the other hand, any temperature nonuniformity leads to a variation in the continued transport of the fuel within the fuel cells; this likewise reduces the efficiency of the fuel cell stack.